As is well known in the art, electro-winning refers to the technique of extracting a metal from its soluble salt by an electrolytic cell. It is used in recovery of zinc, cobalt, chromium, and manganese, and has recently been applied to copper when in the form of a silicate ore. For any specific metal, the salt in solution is subjected to electrolysis and is electro-deposited on a cathode starter plate. In particular, electro-winning techniques used to produce pure metallic copper from leach/solvent electrolytes consist of applying an electrical potential between inert lead alloy anodes and stainless steel or copper cathodes immersed in a CuSO4—H2SO4—H2O electrolyte. Copper metal is deposited at the cathode and oxygen gas released at the anode. Purity of the refined copper can be maximized by, amongst other factors, providing for straight cathodes fabricated from stainless steel arranged vertically in the electrolytic bath and positioned at uniform distances.
Similarly, electro-refining refers to a technique for purifying metals by electrolysis using an impure metal as anode from which the pure metal is dissolved and subsequently deposited at the cathode. In particular, when electro-refining copper, copper is dissolved from impure copper anodes into a CuSO4—H2SO4—H2O electrolyte. Pure copper without the anode impurities is plated onto the cathodes. Copper refined in this manner is of very high purity, typically with less than 20 ppm impurities plus oxygen which is controlled at about 0.025%.
When using cathode starter plates which are manufactured from the same metal as that being refined, once deposited the entire plate can be melted with a portion of the molten refined metal being retained to fabricate new starter plates. When another metal, such as stainless steel, is used to fabricate the starter plate the refined metal deposited on the starter plate must be subsequently removed. Over the years, the electrolytic refining/electro-winning industry has developed a variety of equipment for the mechanized removal of metals deposited on starter plates as a result of the refining process. Currently, two major advanced technologies in the field of permanent cathode technology are used to supply the copper industry. These are Falconbridge's “Kidd Process Technology” and the “ISA Technology 2000 Process” marketed by M.I.M. Technologies (Mount ISA Mines of Australia.) for example.
In order to strip a starter plate covered with refined metal the plate must typically be moved between a number of stations for washing, stripping, refinishing, etc. One problem with moving the plate is the weight of the deposited metal which can be in excess of 300 kg., thereby requiring a robust and rugged structure for moving the plates.
The Falconbridge Kidd Process, which was first developed for the Kidd Creek Refinery in Timmins in 1985, uses a rotary, top driven carousel with cathode plates conveyed through multiple stripping stations. The cathode plates are suspended by hanger bars from supports mounted to the carousel base. One original Kidd patent included such a top driven carousel.
In the Kidd Process, copper deposited on the starter plate is normally stripped in plates (one deposited on each side of the starter plate) joined along a bottom edge. As will be seen below, however, cathode starter plates fabricated from stainless steel typically incorporate a well known “V” groove along the bottom edge which allows the deposited copper plates to be readily separated from one another during stripping. Stripped joined copper plates thereafter drop at least their full length to be removed and are subsequently stacked in bundles.
The ISA Process, which was first developed by Mount ISA Mines for their Townsville Refinery, Queensland, Australia in 1972, utilizes a linear conveyer system, wherein the cathodes are conveyed, supported on a bottom edge, by a narrow pan-type conveyor, through multiple stripping stations. The cathodes are kept vertical by stainless steel round (pipe/tube) guide rails. Initially, the bottom edge of mother blanks were dipped in wax to prevent copper growth around the bottom edge. The copper can then be stripped as two separate halves, unlike the bottom joined plates in the Falconbridge Kidd Process. In the late 90's the technology was upgraded by introducing a “V” notch cathode to eliminate the wax, which acts as an. Impurity, from the process. Additionally, the stripping equipment was redesigned to take advantage of the plane of weakness in the plated copper at the lower edge “V” notch, such that the plates are gripped and rotated from the mother blank to break the bottom joint, thus yielding separate plates.
A number of features are now well mastered in the art, such as moving a cathode plate between stations; moving a cathode plate transversely between stations (i.e. in a direction which is perpendicular to the surface of the cathode plate); suspending a plate from above for transverse movement on a conveyor between stations; using a hook assembly, comprised of a pair of hooks inserted through a pair of rectangular slots, for raising and moving a cathode plate laterally.
Still, many recurrent problems are faced. Indeed, since stripping systems in electrolytic plants process thousands of electrodes daily, a continual effort is being made to increase their reliability and ease of maintenance. Conventionally, electrodes are transferred by a multitude of chain or walking beam-type conveyor designs, which incorporate chain on the conveyor, or structural members linked together and positioned with hydraulic cylinders as a walking beam for conveyance. However, the fluids used in the electrolytic refining process are highly corrosive and therefore it has proven necessary with such prior art designs to use expensive chain made of stainless steel. The heavy weight of the deposited metal combined with the stop and start of the conveyors as they move plates between stations leads to permanent chain stretch and results in positioning problems requiring regular adjustments. Similarly, the corrosive electrolyte accelerates wear of the pins and bushings of conveyors located below the electrodes which is further accelerated as the conveying speed increases.
The rotary carousel type conveying apparatus, such as the one utilized In the Kidd Process Systems mentioned hereinabove, alleviates the positioning problems at the stations as the electrodes supports are rigidly attached to a rotating base unit. However the high mass with great inertia of the structure requires a heavy duty drive unit with its associated high capital cost. Additionally, the rotary carousel is a relatively complex device which has proven difficult to competitively adapt in simpler implementations.
Relatively recent (in the years 2000 and 1997) applications by Outokumpu Oyj (WO 00/77276 and WO 00/18988) and Outokumpu Wenmec Systems (WO 97/24475), for example, address these difficulties by utilizing complicated arrangements of linked structural elements.
It remains that, for higher capacity stripping systems, rapid and reliable transfer of the vertically positioned electrodes between the stripping (working) stations is of paramount importance.
Clearly, there is still a need in the art for an improved high speed linear conveyor system to position plated cathodes in adjacent stations of automated stripping systems in refining/electro-winning plants, such as copper refining/electro-winning plants for example.